1. Field of the Invention
This invention relates to a photo-sensor array for a contact type linear image sensor, and more particularly to a photo-sensor array formed on one surface of a transparent substrate and introducing image light from the opposite surface of the transparent substrate.
2. Description of the Prior Art
In recent years, a contact type image sensor has been developed for facsimile equipment. The contact type image sensor detects images by being directly contacted with the document to be sensed. This means that no lens system is required, resulting in a compact and adjustment-free sensing system.
One such contact type image sensor was reported at the 76th meeting of the Institute of Image Electronics Engineering of Japan (Nov. 21, 1983, No. 83-04-01) under the title "High Speed Amorphous Silicon Contact Type Image Sensor". The image sensor was formed on a transparent insulator substrate. A photo-shield layer having windows for introducing the light to be sensed was formed on one surface of the insulator substrate. A SiO.sub.2 layer was deposited over the photo-shield layer and the exposed surface of the insulator substrate. A common electrode made of a transparent conductor was formed on the limited surface of the SiO.sub.2 layer. Then, an amorphous silicon layer was formed on the transparent conductor and the exposed portion of the SiO.sub.2 layer. Finally, a plurality of individual electrodes were formed on the amorphous silicon layer and extended onto the exposed surface of the SiO.sub.2 layer. Good performance was reported.
However, a considerable leakage current has been observed with this structure according to the inventor's experiments. The inventor has newly found that this leakage current is due to defects in the amorphous silicon layer between the common electrode and the individual electrodes, such as cracks and pin-holes. The lack of flatness of the surface of the SiO.sub.2 layer causes the defects in the amorphous silicon layer. This lack of flatness is due to the existance of the common electrode and the photo-shield layer on the transparent insulator substrate. Particularly, the conventional structure has a wide overlap between the common and individual electrodes. In the overlapped area, a hollow exists on the surface of the SiO.sub.2 layer upon the window of the photo-shield layer. Many defects are generated in the amorphous silicon at the periphery of the hollow, which cause a leakage current between the common and individual electrodes. Thus, the contact type image sensor of the prior art has a large dark current in the imaging output signal.
The aforementioned defects in the amorphous silicon may be overcome to a degree by decreasing the thicknesses of the photo-shield layer and the common electrode. However, a thin photo-shield layer renders the photo-shield effect imperfect, and a thin common electrode produces a high parasitic resistance. Both of these characteristics deteriorate the photo-sensing performance.
Another drawback of the prior art structure is the complex manufacturing process required. Processes for forming the photo-shield layer, the SiO.sub.2 layer and the common electrode are necessary, before the formation of the amorphous silicon layer. Such formation of multiple layers causes contamination in particularly amorphous silicon layer. The surface contamination causes crystal defects in the amorphous silicon layer formed thereon. To avoid such contamination, careful processing is necessary, resulting in low productivity and high production costs.